Known solenoids include a magnetic coil coiled around a bobbin, a stationary pole piece, sometimes known as a core, axially aligned with, and at least partially within the bobbin, and an armature axially aligned within the bobbin and movable between a first position and a second position relative to the core in response to an electrical current applied to the coil. Separating an end face of the core and an end face of the armature is an air gap proportional to the maximum stroke of the solenoid.
When an electrical current is applied to the coil, a magnetic flux path is created which at some point travels across the air gap separating the armature and the core. The strength of the magnetic flux field directly affects the force with which the armature is displaced. Typically, the magnetic flux generated is reduced because of the air gap. The greater the gap, the greater the reduction in the magnetic force. Similarly, the smaller the air gap, the greater the magnetic force.
As the armature moves, the distance between the end surfaces of the core and armature (i.e., the air gap) varies leading to a variable magnetic flux as the armature moves. The performance of the solenoid is affected by the varying flux field in unpredictable in unrepeatable ways. In particular, the force applied by the armature varies with position of the armature in the stroke, even with a constant electrical current applied to the coil.
In some applications, it is desirable to maintain a constant, or near constant, force of the armature at a selected current. Current solenoids cannot achieve this goal because of the varying air gap.
It is often desirable to manufacture solenoids that produce a consistent armature force within a tight tolerance. Accordingly, solenoids that can be easily adjusted, or fine-tuned, to produce a consistent armature force are desirable for many applications.
With decreased available space for various mechanical components, for example solenoids in automotive transmissions, it is often desirable to limit the size of solenoids while maintaining the desirable functions of the solenoid.
Accordingly, a need exists for a compact solenoid that can easily be manufactured to maintain a constant, or near constant, armature force along the stoke length of the armature under a constant electrical current.